Simultaneous removal of nitrate nitrogen and orthophosphate by electroreduction and electrochemical precipitation

Abstract

Electrochemical methods can effectively remove nitrate nitrogen (NO 3 -N) and orthophosphate phosphorus (PO 4 -P) from wastewater. This work proposed a process for the simultaneous removal of NO 3 -N and PO 4 -P by combining electroreduction with electrochemically-induced calcium phosphate precipitation, and its performance and mechanisms were studied. For the treatment of 100 mg L –1 NO 3 -N and 5 mg L –1 PO 4 -P, NO 3 -N removal of 60–90% (per cathode area: 0.25–0.38 mg h –1 cm –2 ) and 80–90% (per cathode area: 0.33–0.38 mg h –1 cm –2 ) could be acquired within 3 h in single-chamber cell (SCC) and dual-chamber cell (DCC), while P removal was 80–98% (per cathode area: 0.10–0.12 mg h –1 cm –2 ) in SCC after 30 min and 98% (per cathode area: 0.37 mg h –1 cm –2 ) in DCC within 10 min. The faster P removal in DCC was due to the higher pH and more abundant Ca 2+ in the cathode chamber of DCC, which was caused by the cation exchange membrane (CEM). Interestingly, NO 3 -N reduction enhanced P removal because more OH – can be produced by nitrate reduction than hydrogen evolution for an equal-charge reaction. For 10 mg L –1 PO 4 -P in SCC, when the initial NO 3 -N was 0, 20, 100, and 500 mg L –1 , the P removal efficiencies after 1 h treatment were < 10%, 45–55%, 86–99%, and above 98% respectively. An increase in Ca 2+ concentration also promoted P removal. However, Ca and P inhibited nitrate reduction in SCC at the relatively low initial Ca/P, as CaP on the cathode limited the charge or mass transfer process. The removal efficiency of NO 3 -N in SCC after 3 h reaction can reduce by about 17%, 40%, and 34% for Co 3 O 4 /Ti, Co/Ti, and TiO 2 /Ti. The degree of inhibition of P on NO 3 -N removal was related to the content and composition of CaP deposited on the cathode. On the cathode, the lower the deposited Ca and P, and the higher the deposited Ca/P molar ratio, the weaker the inhibition of P on NO 3 -N removal. Especially, P had little or even no inhibition on nitrate reduction when treated in DCC instead of SCC or under high initial Ca/P. It is speculated that under these conditions, a high local pH and local high concentration Ca 2+ layer near the cathode led to a decrease in CaP deposition and an increase in Ca/P molar ratio on the cathode. High initial concentrations of NO 3 -N might also be beneficial in reducing the inhibition of P on nitrate reduction, as few CaP with high Ca/P molar ratios were deposited on the cathode. The evaluation of the real wastewater treatment was also conducted.

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